A conventionally proposed refrigerant circuit for an air conditioning system isolates a part of an indoor heat exchanger and switches a four-way valve from a heating cycle to a cooling cycle with refrigerant in the isolated heat exchanger kept at high temperature and high pressure before the operation shifts from heating operation to a defrosting operation, thereby defrosting an outdoor heat exchanger. This refrigerant circuit improves indoor comfort during defrosting (for example, PTL 1: Japanese Patent Laying-Open No. 2012-167860).